Carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur are the primary elements essential to all life. Soils contain all of these elements in addition to other macronutrients and micronutrients which are needed for plant growth. Typically, such elements are not present in the soil in sufficient quantity or in forms that can support maximum plant growth and yield. In order to overcome these deficiencies, fertilizers having specific chemical constituents in specific amounts are often added to the soil, thereby enriching the growth medium. The fertilizers may be supplemented with certain trace elements such as copper, iron, manganese, zinc, cobalt, molybdenum, and boron, as oxides or salts containing the elements in the cationic form.
Agriculturally, metal ions are essential nutrients for plant growth. Soil deficiency because of the unavailability or exhaustion of metal ions is very often the cause of poor plant growth.
In the past, applications of phosphorus have typically been only about 20 percent efficient (that is, only 20 percent of the applied phosphorus is available to the crop in the year of treatment).
Phosphorus is routinely used in starter fertilizers applications. However, most phosphorus is immobile in the soil and subsequently small seedling roots have difficulty obtaining the necessary amounts for rapid growth. For these reasons, phosphorus is routinely used as a starter fertilizer, even when overall phosphorus levels in a field may be adequate or high. However, phosphate in the soil can interact with other nutrients and/or metals and immobilize them. Furthermore, there are obvious environmental concerns regarding high soil phosphate levels leaching into the environment as well as potential toxicity to seeds and plants.
Clearly, methods for reducing the amount of phosphorus applied as fertilizer as well as methods for more effectively and/or efficiently enabling plants to use phosphorus already in the soil are needed.
Plant growth promoting bacteria (PGPB) benefit commercial crops by improving both yields and plant tolerance to stresses (high salinity, drought, etc.). Some PGPB possess other beneficial traits such as bioremediation of hydrocarbon and heavy-metal contaminated soils (Cheng et al. 2007, Albano et al. 2016, Aukema et al. 2014). PGPB can interact with several economically important field crops including canola, soybean, wheat, and corn (Nehra et al. 2015). PGPB can promote higher crop yields and expedited or early crop emergence as well as growth under both stressed and optimal plant conditions (Cheng et al. 2007). This can occur from a variety of mechanisms including nutrient cross-feeding, modulation of plant stress hormones, and assistance in the creation of a beneficial rhizosphere environment to increase nutrient bioavailability (Nehra et al. 2015).
One important group of PGPB, Pseudomonas spp., have been found to modulate the plant stress response in order to improve the plant's tolerance to salinity, petroleum hydrocarbons, and heavy-metal toxicity (Cheng et al. 2007, Greenberg et al. 2007, Albano et al. 2016).
Of the Pseudomonas strains that have been described in the literature, multiple PGP features have been well-characterized at the physiological and molecular levels.
Typically three pathways strongly associated with PGP phenotype in Pseudomonas spp. are: i) IAA (indole-3-acetic acid) biosynthesis by PGPB from tryptophan secreted into the rhizosphere by the plant, uptake of IAA by the plant stimulates growth (Cheng et al. 2007); ii) 1-aminocyclopropane-1-carboxylate (ACC) degradation by the PGPB via an ACC deaminase, high levels of ACC cause a plant to elicit an ethylene production response that causes necrosis of the plant tissue (Cheng et al. 2007); and iii) catabolism of phenyl acetate as a growth substrate that is secreted from the plant into the rhizosphere to promote the growth of specific organisms (Basha et al. 2006). Phenyl acetate can also be consumed from inside the plant by specific bacteria, as such, its catabolism by bacteria can be linked to an endophytic lifestyle (Basha et al. 2006). Endophytic organisms can provide several benefits to the plants including modulation of plant hormones, increasing bioavailability of nutrients, and acting as a biocontrol agent (Parnell et al. 2016).
Typically, PGPB are also capable of solubilizing various forms of insoluble phosphate found in soils. For example, insoluble calcium phosphate by conversion of glucose excreted from the root of the plant to gluconate in the rhizosphere drives down the pH and increases the solubility, and therefore the bioavailability of phosphate near or on the plant roots (Buch et al. 2008). These organisms may also assist in making other forms of phosphate more bioavailable such as phosphate bound to organic material, phosphate bound to metals in the soil, and other forms of fertilizer phosphate such as struvite (Rodriguez et al. 1999). In doing so, PGPB have the potential to increase the availability of phosphate to plants. In most systems this could lead to a reduction of applied phosphate leading to reduced costs and increased yields to grain farmers in general.